As known, the bicycle is a mechanical means moved by muscle force that is transmitted to the rear “driving” wheel through a motion transmission system. It comprises a pair of cranks arms, on which the cyclist exerts a propulsive thrust, one or more driving toothed wheels, set in rotation by direct coupling with the crank arms, and one or more driven toothed wheels or “sprockets”, set in rotation by the driving toothed wheels through a chain, said sprockets being coupled with the hub of the rear wheel.
In particular, racing bicycles comprise a plurality of sprockets of various diameters and a plurality of driving toothed wheels, also of various diameters. The chain simultaneously engages a driving toothed wheel and a sprocket and can be selectively moved onto them through a front gearshift and a rear gearshift, so as to obtain the combination of driving toothed wheel and sprocket that offers the gear ratio most suitable for the travel conditions.
The front gearshift is mounted on the bicycle frame close to the plurality of driving toothed wheels and comprises a derailleur that moves the chain from one driving toothed wheel to another. The rear gearshift is mounted on the bicycle frame close to the plurality of sprockets and comprises a derailleur that moves the chain from one sprocket to another.
In the context of the present description, the bicycle gearshift to which reference is made is the rear one.
The derailleur of the rear gearshift is made according to an articulated parallelogram linkage, in which the sides of the parallelogram are articulated in pairs to one another, along respective substantially parallel rotation axes, through pins. In particular, such an articulated parallelogram is formed from a support body, intended to be mounted on the frame, a so-called chain guide suitable for moving the chain between different engaged positions on respective toothed wheels (the sprockets of the sprocket set), and a pair of articulation arms or connecting rods (generally identified as outer connecting rod and inner connecting rod), which connect the chain guide to the support body in a mobile manner. In the derailleur of the rear gearshift the chain guide comprises a rocker arm group.
Each connecting rod is rotatably connected to the support body and to the chain guide through respective articulation pins inserted in respective holes.
The derailleurs can be manually actuated (such derailleurs are also known as mechanical derailleurs) or motor actuated (such derailleurs are also known as electrically commanded derailleurs).
In the articulated parallelogram linkage of the rear gearshift two chain tensioning springs (generally of the helical type) are typically inserted, which work antagonistically to determine the arrangement of the chain guide when engaged with the chain.
Generally, the chain tensioning springs are provided in the support body and in the rocker arm group of the chain guide.
The chain tensioning spring of the support body also has the function of absorbing the bumps that the rear gearshift is inevitably subjected to during use, thus avoiding damage or imprecisions in gearshifting of the rear gearshift.
To mount the derailleur of the rear gearshift on the bicycle frame, a mounting device is used comprising:                a pin provided with a threaded end portion, which is screwed into a corresponding hole of the bicycle frame;        the support body of the derailleur;        a through hole, formed on the support body and that is crossed by the pin; and        an abutment element associated with the pin in an annular seat thereof, in which the abutment element closes the support body on the pin, i.e. provides a support in the axial direction to the support body on the pin.        
An abutment element used in the prior art is an elastic stop ring, typically made from spring steel, the circumferential extension of which is not complete and that is provided, at the two opposite ends, with holes for the engagement of a suitable assembling/disassembling caliper of the elastic stop ring in/from the annular seat.
The Applicant has found that a mounting device of the type described above can undergo improvement in terms of structural strength, ease of assembling and practicality of maintenance.
The technical problem at the basis of the present invention is that of providing a device for mounting a derailleur of a rear gearshift on a bicycle frame in which the abutment element—which provides a support in the axial direction to the support body on the pin—has a greater structural strength than that of the elastic stop ring of the prior art described above, and can be easily mounted also by acting manually, as well as can be rapidly dismounted for maintenance of the bicycle gearshift.